Modern mobile devices may include sensors such as optical sensors, which are used to measure biometric information. For example, a photoplethysmogram (PPG) sensor obtains volumetric measurements of blood vessels near the skin surface. When the heart pumps blood, the resulting pressure pulse causes changes to blood vessels. The pressure pulse may distend arteries and arterioles in skin tissue. An optical sensor, such as a PPG sensor, may be used to detect a change in blood vessel volume caused by the pressure pulse. Blood vessel volume change caused by the pressure pulse may be detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Blood vessel volume change caused by blood flow to the skin may be modulated by various physiological parameters. Therefore, the information provided by PPG sensors may be used to obtain a variety of biometric measurements.
One drawback of blood volume sensors, such as PPG sensors, is that the biometric measurements obtained are sensitive to user movement. Thus, user movements can affect optical sensor measurements and introduce noise and other artifacts into the measured signals. Some techniques to address noise may use adaptive filters, which typically reduce noise based on motion related signals provided by an accelerometer. Adaptive filters use the motion related signals to compensate for motion induced noise in the optical sensor determined heart rate signal. However, for many activities, a higher activity rate may result in greater energy being expended and consequently a faster heart rate (e.g. more beats per minute). Thus, in many situations, heart rate may track the frequency of motion.
For activities with regular motion, the temporal frequencies related to the motion are present in the spectrum of both accelerometers and PPG sensors. The term “heart rate equal cadence” is used to refer to instances where a spectral component of the motion occurs at the same temporal frequency as a component of a cardiovascular parameter such as the heart rate. When heart rate equal cadence occurs, a separate heart rate signal may not be discernible, preventing heart rate estimation.